Page 27 - Boiler_Operators_Handbook,_Second_Edition

12 Boiler Operator’s Handbook

3 to 2 turndown must be a novice in the industry) mini- ing (looking along the length of an edge with your eye
mum operating rate is determined by dividing the larger close to it) is the best way to check to confirm an edge is
number into one. If you run into the nut that described a straight.
3 to 2 turndown then the minimum capacity is 2/3 of full For measures larger than something you can check
capacity. Divide the large number into one and multiply with that square you should use a 3 by 4 by 5 triangle;
by 100 to get the minimum firing rate in percent. the same thing the Egyptians used to build the pyra-
We also use the term “load” when describing mids. You lay it out by making three arcs as indicated in
equipment operation. Load usually refers to the demand Figure 1-1. You frequently also need a straight edge as
the facility served places on the boiler plant but, within the reference that you’re going to be square to, in which
the correct context, it also implies the capacity of a piece case you mark off 3 units along that edge to form the one
of equipment to serve that load. If we say a boiler is side, that’s drawing the arc to find the point B by mea-
operating at a full load that means it is at its maximum; suring from point A. An arc is made 4 units on the side at
half load is 50%, etc. point C by measuring from point A then another arc of 5
A less confusing but more difficult measure to units is made measuring from point B and laying down
address are “implied” measures. Some are subtle and an arc at D. Where the A to C and B to D arcs cross (point
others are very apparent. A common implied measure E) is the other corner of the 3 by 4 by 5 triangle and side
in a boiler plant is half the range of the pressure gauge. A to B is square to A to E. The angle in between them is
Engineers normally select a pressure gauge or thermom- precisely 90 degrees.
eter so the needle is pointing straight up when the sys- The beauty of the 3 by 4 by 5 triangle is the units
tem is at its design operating pressure or temperature. can be anything you want as long as the ratio is 3 to 4 to
We always assume that the level in a boiler should be 5. Use inches, or even millimeters, on small layouts, and
at the center of the gauge glass, that’s another implied feet on larger ones. If you were laying out a new storage
measurement. In other cases we expect the extreme of shed you might want to make the triangle using 30 feet,
the device to imply the capacity of a piece of equipment; 40 feet, and 50 feet. It’s difficult to get more precise, even
steam flow recorders are typically selected to match the if you’re using a transit.
boiler capacity even though they shouldn’t be. The prob- Another challenge is finding a 45 degree angle.
lem with implied measurements is that we can wrongly The best solution for that is to lay out a square side to
assume they are correct when they’re not. Keep in mind get that 90 degree angle then divide the angle in half.
that someone could have replaced that pressure gauge Figure 1-2 shows the arrangement for finding half an
with something that was in stock but a different range. angle. Simply measure from the corner of the angle out
I failed to make that distinction one day and it took two to two points (C and D) the same distance (A to B) then
hours of failed starts before I realized the gauge must be draw two more arcs, measuring from points C and D a
wrong and went looking for the instruction book. Yes, distance E, and F identical to E to locate a point where
I’ve done it too. the arcs cross at G. A line from A to G will be centered
Probably one of the most common mistakes I’ve
made, and that I’ve seen made by operators and con-
struction workers, is not getting something square. All
too often we’ll simply eyeball it or use an instrument
that isn’t adequate. The typical carpenter’s square, a
piece of steel consisting of a two foot length and sixteen
inch length of steel connected at one end and accepted
as being connected at a right angle works well for small
measurements but using it to lay out something larger
than four feet can create problems. I say “accepted as
being square” because I’ve used more than one of them
to later discover they weren’t. Drop a carpenter’s square
on concrete any way but flat and you’ll be surprised
how it can be bent. On any job that’s critical, always
check your square by scribing a line with it and flipping
it over to see if it shows the same line. Of course the
one side you’re dealing with has to be straight. Eyeball- Figure 1-1. Creating a right angle